Automatic truss jig setting system

ABSTRACT

A automatic truss jig setting system is disclosed that includes a table including a plurality of segments with a side edge of adjacent segments defining a slot. At least one pin assembly, and optionally a pair of pin assemblies, is movable independently of each other along the slot. Movement apparatus is provided for independently moving the pin assemblies along the slot. Each of the side edges of the segments associated with the slot defines a substantially vertical plane with a zone being defined between the substantially vertical planes of the side edges, and the movement apparatus is located substantially outside of the zone of the slot. The invention may optionally include a system for handling the obstruction of pin assembly movement, and a system for keeping track of the position of the pin assembly when the pin assembly has encountered an obstruction.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/369,038, filed Feb. 18, 2003 now U.S. Pat. No. 6,712,347.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to jig systems and more particularlypertains to a new automatic truss jig setting system for setting andresetting jig stops in a highly efficient and effective manner.

2. Description of the Prior Art

Jig systems have been used to hold building elements, such as woodboards, in proper position while the building elements are attached toeach other to construct a roof support truss. Known jig systemstypically employ a horizontal surface (such as a table) for resting thebuilding elements thereon and a plurality of adjustable stops forindicating the proper positions of the building elements in the desiredtruss design and for holding the building elements in those positionsuntil the elements can be secured together in a permanent manner. Foreach different truss design, the stops must be repositioned on the jigsurface to reflect the different positions of the building elements.Computer programs have been developed to calculate, for various trussdesigns, the positions of the stops from a reference line, such as anedge of the table. Conventional practice has been to measure thepositions of the stops from the reference line, manually move the stopsto the positions, manually secure the stops in the desired positions,place the building elements on the table against the stops, fasten thebuilding elements together, remove the completed truss, and then repeatthe process by releasing and then re-securing the stops for eachdifferent truss design.

As there can be significant variation between the size and shape of roofsupport trusses used for the same building, a significant amount of thetruss production time has been dedicated to resetting the positions ofthe stops, especially when only one or two trusses for each truss designare needed. For example, the setup for positioning the stops on thetruss jig may take approximately 15 minutes or more, while the timeneeded to actually construct the truss may be only 3 minutes.

Various approaches have been used to speed up the jig stop set upprocess, and one approach has been to project an image of the desiredtruss in actual shape and size on the surface of the jig, which can helpminimize the amount of measurement required but does not eliminate theneed to repeatedly secure and loosen the stops for each truss design.Further, the projection equipment and associated controlling systemtends to be relatively expensive.

Another approach has been to employ a system that automatically movesthe stops (sometimes referred to as “pucks”) along slots in thehorizontal surface of the jig. While in concept these systems can savetime otherwise needed to measure, move and secure the stops on the jig,there have been problems that have cropped up with these systems thatmake them less time saving and reliable as they could be for optimumefficiency.

For example, the environment in which the jig systems are used is filledwith debris and dust. Even when the building elements are cut and shapedat a location remote from the jig system, the building elements oftencarry sawdust and wood chips onto the surface of the table of the jigsystem. This debris falls or is pushed into the slot in which the puckmoves. As each puck is typically mounted on a screw-threaded rod that ispositioned below the puck in the slot, the debris often falls onto therod. Since the rod rotates to move the puck, a rod caked with debris canhamper and even prevent movement of the puck along the rod. Thus,regular and frequent cleaning of the rod is needed to minimize thepossibility of breakdowns of the system.

Further complicating this situation is the fact that the screw-threadedrods typically are covered with some type of lubricant to facilitatemovement of the puck along the rod, and this often sticky lubricantholds the debris on the surface of the rod. The encrusted rod can carrythe debris into the cooperating parts of the system, and causeadditional friction and failure.

Still further exacerbating this problem in the known systems is theplacement of the rod in a channel located below the slot with a closedbottom that holds the debris in close proximity to the rod, so thatinfrequent clearing of the channels can bring debris in contact with therods from the bottom (as well as from the top as debris falls from thetable surface).

Still further, the known systems lack a suitable system for dealing withencounters between the puck and an obstruction while the puck is movingto the desired position. Some known systems permit slippage between thedriving motor and the rod when resistance in moving the puck isencountered, but the slippage results in the system losing track of theposition of the puck on the jig and the system must then be reset insome manner so that the system can reassess the position of the puck onthe table and reposition each of the affected pucks to the correctpositions. This is particularly a problem in systems that rely upon theprecise rotation of the motor (such as a stepper motor) in order todetermine the current position of the puck on the jig system. Anyslippage between the motor and the rod results in the puck being in aposition different from where the system registers the location of thepuck.

Thus, while the known systems for automatically positioning the stops onthe jig are an improvement over jig systems requiring manual positioningof the stops, there are significant new problems that have arisen withthe use of these automatic systems that hamper their operation in ahighly efficient and effective manner.

SUMMARY OF THE INVENTION

In view of the foregoing disadvantages inherent in the known types ofjig systems known in the prior art, the present invention provides a newautomatic truss jig setting system that is highly suitable for settingand resetting jig stops in a highly efficient and effective manner.

To attain this, in one aspect of the invention, the jig setting systemcomprises a table including a plurality of segments with side edges ofadjacent segments defining a slot. A plurality of pin assemblies ismovable independently of each other along the slot. Movement apparatusis provided for independently moving the pin assemblies along the slot.Each of the side edges of the segments associated with the slot definesa substantially vertical plane with a zone being defined between thesubstantially vertical planes of the side edges, and the movementapparatus is located substantially outside of the zone of the slot.

In another aspect of the invention, the jig setting system comprises atable including a plurality of segments with side edges of adjacentsegments of the plurality of segments defining a slot. A pin assembly ismovable along the slot. Movement means is provided for moving the pinassembly along the slot. The movement means includes rotation means forproducing rotational motion, converting means for converting rotationalmotion into translational motion by the pin assembly, rotationtransferring means for transferring rotational motion of the rotationmeans to the converting means while permitting slippage between therotation means and the converting means when translational movement ofthe pin assembly is resisted, and position sensing means for sensingrotation of the converting means to determine a position of the pinassembly along the slot. Sensing of rotation of the converting means bythe position sensing means is not affected by any slippage of therotation transferring means.

An optional aspect of the invention includes detecting means fordetecting interference with movement of one of the pin assemblies. Thedetecting means may comprise means for determining when rotationalmotion by the rotation means is not transferred to the converting meansby the rotation transferring means, means for temporarily delaying for apredetermined time period further actuation of the rotation means whenrotational motion by the rotation means is not transferred to theconverting means by the rotation transferring means, and means forreinitiating rotational movement of the rotation means when thepredetermined time period has passed. The detecting means may alsocomprise means for canceling further movement of the pin assemblies if,after a preset period of time, attempts to reinitiate rotationalmovement of the rotation means does not result in rotational motion bythe rotation means being transferred to the converting means by therotation transferring means.

There has thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are additionalfeatures of the invention that will be described hereinafter and whichwill form the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein are for the purpose of description and should not beregarded as limiting.

As such, those skilled in the art will appreciate that the conception,upon which this disclosure is based, may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

One significant advantage of the present invention is the ability tominimize, if not altogether prevent, the buildup of debris on (or in theproximity of) the apparatus for moving the pin assemblies so thatfrequently cleaning and sudden breakdowns of the apparatus can beavoided. Another significant advantage is the ability of the system tohandle situations where one of the pin assemblies is prevented frommoving by an obstruction.

Further advantages of the invention, along with the various features ofnovelty which characterize the invention, are pointed out withparticularity in the claims annexed to and forming a part of thisdisclosure. For a better understanding of the invention, its operatingadvantages and the specific objects attained by its uses, referenceshould be made to the accompanying drawings and descriptive matter inwhich there are illustrated preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects of the inventionwill become apparent when consideration is given to the followingdetailed description thereof. Such description makes reference to theannexed drawings wherein:

FIG. 1 is a schematic top view of a new automatic truss jig settingsystem according to the present invention employed on a portion of a jigtable.

FIG. 2 is a schematic sectional view of the present invention positionedalong a slot of the jig table.

FIG. 3 is a schematic top view of the jig table with portions of thetable removed to expose detail of the present invention.

FIG. 4 is a schematic end view of the present invention particularlyillustrating the assembly for rotating the rods.

FIG. 5 is a schematic side view of the assembly for rotating the rods.

FIG. 6 is a schematic sectional view of an optional configuration of thepresent invention.

FIG. 7 is a schematic diagram of operational elements of the presentinvention.

FIG. 8 is a schematic flow chart of an aspect of the operation of thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference now to the drawings, and in particular to FIGS. 1 through8 thereof, a new automatic truss jig setting system embodying theprinciples and concepts of the present invention and generallydesignated by the reference numeral 10 will be described.

As best illustrated in FIGS. 1 through 8, the truss jig positioningsystem 10 of the invention may suitably be employed on a table 12 thathas and defines a support plane 14 on which work pieces or buildingelements (such as wood boards or other building materials) are supportedin proper position for forming a structure such as a support truss for aroof of a building. The table 12 may comprise a plurality of segments 16that have upper surfaces 15 that substantially lie in and define thesupport plane 14 of the table. The upper surface of each of the segmentsmay be substantially planar, and a plane of the segments may be orientedsubstantially horizontal.

The segments 16 of the table are separated by slots 18, and preferablyeach of the slots extend substantially parallel to each other on thetable. Each of the slots 18 may extend substantially perpendicular tothe length of the table, and may extend across, or substantially across,the width of the table. In such a configuration, the slots may beoriented substantially parallel to the rise (or height) dimension of atruss when the truss is rested on the support plane of the table.

Each of the slots 18 may be defined by opposing side edges 20, 22 ofadjacent segments 16, 17 of the table. Each of the side edges 20, 22that are associated with one of the slots may define a substantiallyvertical plane that extends along the respective side edge. The spacebetween the opposing side edges 20, 22 of adjacent segments 16, 17 maybe considered to define a zone 24 that lies between the side edges andextends downwardly from the slot between the segments. In one embodimentof the invention, the zone 24 is substantially free of structure thatwould catch debris falling from the table 12 through the slot 18 andhold the debris close to the slot (and the movement means describedbelow) so that the debris collects and can affect and degrade theperformance of the movement means. It should be recognized that in suchan embodiment of the invention, various structural elements may crossthe zone, but no wall or plate extends across the zone 24 proximate tothe movement means.

A side channel 26, 27 may be provided that opens into the slot 18 frombelow each of the segments 16, 17. Each of the side channels has anopening 28, 29 that lies adjacent the zone 24. The opening 28, 29 maylie in a plane, and the plane of each of the openings may besubstantially vertically oriented. Each of the side channels 26, 27 mayinclude an upper portion 30, an intermediate portion 32, and a lowerportion 34. The upper portion 30 may extend at least a portion of thedistance between adjacent slots 16, 17 in the table 12. The upperportion 30 may comprise a plate member, or a portion thereof, thatextends in a substantially horizontal plane and forms the segment 16,17. The intermediate portion 32 may support the upper portion, and maycomprise a plate member that extends in a substantially vertical plane.The intermediate portion 32 may be positioned substantially opposite ofthe opening 28, 29 of the respective side channel 26, 27. The lowerportion 34 may be positioned below the upper portion 30, with theintermediate portion 32 extending between the lower portion and theupper portion 30. The lower portion 34 may comprise a plate member thatextends in a substantially horizontal plane. The upper 30, intermediate32, and lower 34 portions may thus collectively define the side channel,with the upper portion forming a top wall of the side channel, theintermediate portion forming a back wall of the side channel, and thelower portion forming the bottom wall of the side channel. The opening28, 29 of the side channel 26, 27 may extend between the top and bottomwalls.

The system 10 of the invention includes a pin assembly 40 that ismovable along one of the slots 18 in the table 12. In one highlypreferred embodiment of the invention, a pair of pin assemblies 40, 41are independently movable in a slot 18. Optionally, a plurality of theslots 18 of the table 12 may each have one or more of the pin assembliesassociated therewith. Each of the slots of the table may have pinassemblies, or the pin assemblies may be associated only with everyother slot, or every third or fourth slot of the table, for example.

The pin assembly 40 of the invention may include a pin carriage 42 thatis moveable along, and optionally in, the slot 18. The pin carriage 42may be located at least partially in the zone 24 defined between theadjacent segments 16, 17, and portions of the pin carriage may extendinto each of the opposing side channels 26, 27 associated with the slot.The pin assembly 40 may also include a pin 44 that is mounted on the pincarriage 42 and extends through the slot 18 to a level located above thesupport plane 14. The pin 44 may be substantially cylindrical, and mayhave a substantially uniform diameter along its length. The pin 44 mayalso be elongate with a longitudinal axis that extends in asubstantially vertical direction. It will be appreciated by those ofskill in the art that the pin carriage and the pin may be formed of onepiece of material, however, employing separable parts for this assemblypermits replacement of damaged pins if needed. Further, the pin carriagemay have a plurality of pin mountings so that the pins may berepositioned on the carriage or interchanged when one mounting becomesdamaged.

The pin carriage 42 may have a first passage 46 that extends through thepin carriage with an interior surface 48 that may be threaded with asuitable thread for transferring power, such as, for example, an acme orsquare thread. The pin carriage 42 may also have a second passage 49that extends through the carriage. In one preferred embodiment of theinvention, two of the pin assemblies 40, 41 are associated with eachslot 18 of the table 12. The first 46 and second 49 passages may beoriented substantially parallel to each other in the pin carriage 42.

The system 10 of the invention also includes means for moving the pinassembly 40, or more than one of the pin assemblies 40, 41, along theslot 18. The movement means is preferably located in the side channel orchannels 26, 27 of the adjacent segment 16, 17 of the table 12, and arepreferably located out of the zone 24 associated with the slot 18. As aresult, debris falling through the slot 18 is less likely to contact themovement means, and thus interfere with its operation.

In one embodiment of the invention, the movement means includes meansfor converting the rotational motion into translational motion by thepin assembly, which may include a rod 50 that extends along a portion ofthe slot 18 of the table. Significantly, the rod 50 is nested in theside channel 26 outside of the zone 24 of the slot, so that debrisfalling from the support plane of the table downwardly through the slot18 and into the zone 24 is less likely to land on or otherwise come intocontact with the rod, which is thus shielded by the segment of thetable. The rod 50 may extend transversely with respect to thelongitudinal length of the table.

A first rod 50 may be located in a first one 26 of the side channels anda second rod 51 may be located in a second one 27 of the side channels.The rods are preferably mounted or otherwise supported on the table 12,and may be horizontally spaced to opposite sides of the zone 24 with therods oriented substantially parallel to each other and rotatable aboutsubstantially horizontal axes. Each of the rods 50, 51 may be externallythreaded for engaging the threaded interior surface of the first passage46 of the pin carriage such that rotation of the rod in a firstrotational direction causes translation motion of the pin carriage in afirst direction along the slot 18 and rotation of the rod in a secondrotational direction causes translation motion of the pin carriage in asecond direction along the slot 18. The external threads of the rods 50,51 should be compatible with the internal threads on the pin carriagesuch that power may be transmitted between the respective rod and pincarriage. The threads should thus be of a type suitable for powertransmission, such as, for example, an acme or square thread for power.

The means for moving the pin assemblies may further include rotationmeans for producing rotational motion to rotate each rod 50. In oneembodiment of the invention, the rotational means comprises a motor 52.The motor 52 may be fixedly mounted with respect to the table andpositioned at a location beneath the segments of the table, close to oneend of the respective rods 50, 51 of a slot 18 being driven by themotor. A motor 52 is provided for each rod 50, 51 such that each motorcan be individually actuated to move the associated pin assemblyindependent of other pin assemblies.

The means for moving the pin assemblies may also include rotationtransferring means for transferring the rotational motion of therotation means, such as the motor 52, to the converting means, such asthe rod 50, 51. Significantly, the rotation transferring means permitsslippage between the rotation means and the converting means whenconversion of rotational to translational movement by the convertingmeans is resisted, such as when the moving pin assembly 40 encounters anobstruction on the upper surface of the table or encounters anothermoving or stationary pin assembly 41 located in the slot. The rotationtransferring means may comprise an endless belt 54 wrapped about a pairof pulleys 56, 57, with a first one 56 of the pulleys being mounted onthe motor 52 and a second one 57 of the pulleys being mounted on the rod50, or being linked to the rod in a manner that does not permit slippagebetween the second pulley 57 and the rod 50. The belt 54 allows slippagebetween the pulleys 56, 57 and the belt 54 so that if the rod 50 isunable to turn because the pin assembly 40 has encountered anobstruction, the belt is able to slip with respect to one or both of thepulleys so that the inability to turn the rod 50 does not significantlyimpede the operation of the motor and the motor overheats or becomesdamaged.

In one embodiment of the invention, a driven gear 60 is mounted on therod 50, and a driver gear 62 is mounted on the second pulley 57. Anidler gear 64 may be employed between and be in communication withdriver 62 and driven 60 gears for transferring rotation therebetween,while minimizing the size of the driven gear on the rod 50. Thisstructure transfers rotation between the rod 50 and the second pulley 57so that rotational slippage therebetween is substantially prevented, andthus the rotation of the second pulley accurately represents therotation of the rod.

The means for moving the pin assemblies may also include a positionsensing means for sensing rotation of the converting means withoutslippage between the position sensing means and the converting means, sothat slippage between the rotation means and the converting means doesnot affect the ability of the position sensing means to accurately sensethe position of the pin assembly along the slot. The position sensingmeans may comprise a position sensor 66 that is mounted on the secondpulley 57, or an axle associated with the second pulley. The positionsensor 66 is thus able to sense rotation of the rod 50 without theslippage of the belt interfering with or interrupting the detection ofthe movement of the pin assembly 40.

The rods 50, 51 of the system are located outside of the zone 24 definedby the slot 18, and a substantial portion of the remainder of theapparatus utilized to move the pin assemblies is also removed from thezone 24. It will be noted that while some insubstantial portions of theapparatus utilized for moving the pin assembly or assemblies may impingeupon the zone 24, these portions of the apparatus are generally locatedat the ends of the slot 18 and segments 16, 17 (adjacent lateral edgesof the table 12) where significantly less debris is likely to fall offof the table through the slot. For example, the apparatus generallyshown in FIGS. 4 and 5 is preferably located near an end of the slot 18where positioning of building elements is less frequent. As a result,debris falling from the table 12 through the slot 18 is much less likelyto contact (and perhaps become stuck on) the apparatus for moving thepin assemblies.

The first passage 46 of the pin carriage 42 forms a driver passage suchthat the rod 50 extending through the first passage is able to drive andmove the pin carriage of the pin assembly along the slot. In theillustrative embodiment of the invention, the threaded interior surface48 of the driver passage engages the threaded exterior of the rod 50 sothat rotation of the rod in a first direction causes movement of the pinassembly in one direction and reversing of one of the rods. The secondpassage 49 of the pin carriage 42 forms a slider passage through whichthe rod 51 is slidable without rod 52 engaging or affecting the pincarriage engaging. Thus, the first pin assembly 40 is thus moved by therotation of the first rod 50, while the first pin assembly 40 passesover the second rod 51 without hindering movement of the first pinassembly 40. Conversely, the second pin assembly 41 is moved by thesecond rod 51 as the second rod 51 passes through the driver passage ofthe pin carriage 43 of the second pin assembly 41, while the first rod50 passes through the slider passage of the second pin assembly 41 andis unaffected by the rotation of the first rod 50.

In an optional embodiment of the invention (see FIG. 6), a pin carriage68 may be employed that includes additional slider passages 70, 72 forthe passage of additional rods employed on a modified positioningsystem. The employment of additional rods with the modified pin carriage68 permits the movement of more than two pin assemblies along the samecommon slot, with each of the pin assemblies being driven by its ownrod. This embodiment of the invention can be especially suitable forusing a pair of the pin assemblies to pinch or otherwise trap a buildingelement therebetween, such as a board, and with four (or optionallymore) pin assemblies positioned along a slot, one, two or even moreboards may each be secured by a pair of the pin assemblies.

The system of the invention also may include controlling means forcontrolling the movement of the pin assemblies on the table, sensing themovement of the pin assemblies and monitoring the current positions ofthe pin assemblies at each set up. Software application programs aregenerally available from various sources (such as truss hardwarevendors) for calculating the positions of the stops on a jig table asmeasured from a reference line, such as the edge of the table or othermark. Such programs output sets of coordinates that are used to measurefrom the reference edge of the table to the appropriate position of thestop or stops for each slot. One example of such a truss designapplication program is sold under the tradename MITEK 2000 JIGSET,available from Mitek Industries, Inc. of Chesterfield, Mo., USA,although it should be understood that other similar programs areavailable and may be employed. The controlling means of the system 10may be used to convert the measurements outputted by the applicationprogram (which may be in metric or U.S. measurement units or anysuitable increment) to an appropriate number of position countsrepresenting intervals of movement by the pin assembly along the slot 18in the table.

Upon transfer of the pin assembly positional information to the system,the system actuates, or supplies power to, the respective motors tocause movement of the pin assemblies toward the desired positions in thetruss jig set up. The position sensors detect and count the movement ofthe rods, and when the associated position sensors detect that thenumber of position counts counted correspond to the final position ofthe pin assembly in the set up, the respective motors are deactuated byceasing the supply of power to the motors. In one implementation of theinvention, the movement of the pin assemblies may be performed at morethan one speed, with the, for example, the pin assembly being moved at afirst, relatively higher speed at initial movement and then being movedat a second, relatively lower speed as the pin assembly approaches thedesired position.

If, during the movement of the pin assemblies, one of the pin assembliesencounters an obstruction on the table and the movement of the pinassembly to the desired position along the slot is prevented, the rodwill stop turning and the second pulley will also stop turning as aresult of the non-slipping linkage between the rod and the secondpulley. As the position sensor is associated with the second pulley, anystoppage of the movement of the rod results in the ceasing of thedetection of further counts. However, the motor may continue to turn,and the belt may slip on the first and second pulleys, since theposition sensor has not detected that the pin assembly has reached thedesired position. If the system detects that the supply of power to themotor does not result in a change in the count by the position sensor(see FIG. 8), then the system interrupts power to the motor for apredetermined time period, and then re-supplies power to the motor. Ifagain the actuation of the motor does not result in the advance of thecount by the position sensor, the system again removes power from themotor. This may be repeated over a preset period of time, or optionallyfor a preset number of times, and then the system may signal an errorand cease supplying power to the motor until the system is reset. In oneillustrative implementation of the invention, the system pulses power tothe motor on a 25% duty cycle for a period of approximately 40 seconds,and waits for a count to be returned by the position sensor. Thisprocedure may be executed by a programmable logic control or processor.

With respect to the above description then, it is to be realized thatthe optimum dimensional relationships for the parts of the invention, toinclude variations in size, materials, shape, form, function and mannerof operation, assembly and use, are deemed readily apparent and obviousto one skilled in the art, and all equivalent relationships to thoseillustrated in the drawings and described in the specification areintended to be encompassed by the present invention.

Therefore, the foregoing is considered as illustrative only of theprinciples of the invention. Further, since numerous modifications andchanges will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and operationshown and described, and accordingly, all suitable modifications andequivalents may be resorted to, falling within the scope of theinvention.

1. A truss jig positioning system comprising: a table having a supportplane on which work pieces are supported, the table comprising aplurality of segments, side edges of adjacent segments of the pluralityof segments defining a slot; a pair of pin assemblies movableindependently of each other along the slot; movement means forindependently moving the pin assemblies along the slot; and detectingmeans for detecting interference with movement of one of the pinassemblies.
 2. The system of claim 1 wherein the movement meanscomprises: rotation means for producing rotational motion; convertingmeans for converting rotational motion into translational motion by oneof the pin assemblies; and rotation transferring means for transferringrotational motion of the rotation means to the converting means.
 3. Thesystem of claim 2 wherein the rotation transferring means is capable ofpermitting slippage between the rotation means and the converting meanswhen translational movement of the pin assembly is resisted.
 4. Thesystem of claim 2 wherein the detecting means comprises: means fordetermining when rotational motion by the rotation means is nottransferred to the converting means; and means for temporarily delayingfurther actuation of the rotation means when rotational motion by therotation means is not transferred to the converting means.
 5. The systemof claim 4 additionally comprising means for reinitiating rotationalmovement of the rotation means.
 6. The system of claim 5 wherein themeans for reinitiating rotational movement of the rotation means isdelayed for a predetermined time period.
 7. The system of claim 4additionally comprising means for canceling further movement of the pinassemblies after a preset number of attempts to reinitiate rotationalmovement of the rotation means does not result in rotational motion bythe rotation means being transferred to the converting means.
 8. Thesystem of claim 1 wherein the movement means comprises: a rodoperatively coupled to one of the pin assemblies; a motor; a driverpulley mounted on the motor for being rotated by the motor; a drivenpulley; a belt connecting the driven pulley to the driver pulley fortransferring rotation of the driver pulley to the driven pulley in amanner permitting rotational slippage between the belt and one of thepulleys if rotation of the driven pulley is resisted; means foroperatively coupling the driven pulley to the rod in a manner preventingrotational slippage between the driven pulley and the rod such thatrotation of the driven pulley directly corresponds to rotation of therod; and a position sensor operatively connected to the driven pulleyfor sensing rotation of the driven pulley and thereby sending rotationof the rod.
 9. The system of claim 8 wherein the detecting meanscomprises: means for determining when rotation by the motor is nottransferred to the rod; means for temporarily delaying further actuationof the motor when rotation by the motor is not transferred to the rod;means for reinitiating rotation of the motor.
 10. The system of claim 9wherein further actuation of the motor is delayed for a predeterminedtime period.
 11. A truss jig positioning system comprising: a tablehaving a support plane on which work pieces are supported, the tablecomprising a plurality of segments, side edges of adjacent segments ofthe plurality of segments defining a slot; a pair of pin assembliesmovable independently of each other along the slot; and a pair of rodsfor moving the pair of pin assemblies, each of the rods moving one ofthe pin assemblies along the slot independently of the other of the pinassemblies; wherein each of the rods is positioned below one of theadjacent segments of the table that define the slot.
 12. The system ofclaim 11 wherein a first one of the pair of rods is located below afirst one of the adjacent segments of the table defining the slot, and asecond one of the pair of rods is located below a second one of theadjacent segments of the table defining the slot.
 13. The system ofclaim 11 wherein each of the side edges of the adjacent segments of thetable associated with the slot defines a substantially vertical planewith a zone being defined between the substantially vertical planes ofthe side edges, each rod of the pair of rods being located outside ofthe zone of the slot.
 14. The system of claim 11 wherein a side channelis formed below each of the side edges of the adjacent segments, eachrod of the pair of rods being nested in one of the side channels of theadjacent segments of the table.
 15. The system of claim 11 additionallycomprising position sensing means for sensing rotation of one rod of thepair of rods to determine a position of one of the pin assemblies alongthe slot.
 16. The system of claim 11 additionally comprising detectingmeans for detecting interference with movement of one of the pinassemblies.
 17. The system of claim 11 additionally comprising: a motor;a driver pulley mounted on the motor for being rotated by the motor; adriven pulley; a belt connecting the driven pulley to the driver pulleyfor transferring rotation of the driver pulley to the driven pulley in amanner permitting rotational slippage between the belt and one of thepulleys if rotation of the driven pulley is resisted; means foroperatively coupling the driven pulley to one rod of the pair of rods ina manner preventing rotational slippage between the driven pulley andthe rod such that rotation of the driven pulley directly corresponds torotation of the rod; and a position sensor operatively connected to thedriven pulley for sensing rotation of the driven pulley and therebysending rotation of the rod.
 18. The system of claim 11 wherein the pairof rods comprises a first and second rod, the first rod having athreaded exterior surface; and wherein at least one of the pinassemblies comprises: a pin carriage moveable along the slot; a pinmounted on the pin carriage and extending through the slot above thesupport plane; a first passage extending through the pin carriage withthe first rod passing through the first passage, the first passagehaving an interior surface adapted to engage the threaded exteriorsurface of the first rod such that rotation of the first rod producesmovement of the pin carriage; and a second passage through the pincarriage with an interior surface that is adapted to permit freemovement of the second rod through the second passage when the first rodcauses movement of the pin carriage.
 19. The system of claim 14 whereineach of the side channels comprises a top wall, a back wall, and abottom wall with the opening extending between the top and bottom walls.20. The system of claim 11 wherein the pair of pin assemblies arecapable of pinching a building element therebetween for locating aposition of the building element on the table.